As we see with in "Flight to the Ford". Mortals have minimal presence in the spiritual plane. Aragorn has a little, because of his distant elven ancestry. Legolas would have more, but, being Sindar not Noldor, not enough. The rest of the party have none at all (apart from Frodo, who could hypothetically have used the ring, had he had the skill and knowledge).

Glorfindel, Elrond, or Galadriel would have been able to fight the Balrog at least as effectively as Gandalf, since they are all fully present on the spiritual plane.

So, it's not a matter of raw power, it's simply a matter of Gandalf being the only person able to meet the Balrog on the spiritual plane. Against purely physical opponents, Aragorn, Boromir, Legolas and Gimili are more powerful than Gandalf, as seen against the cave troll. A Far Dragon is the best kind...